In producing hydrocarbons or the like from loosely or unconsolidated and/or fractured subterranean formations, it is not uncommon to produce large volumes of particulate material (e.g. sand) along with the formation fluids. These particulates routinely cause a variety of problems which result in added expense and substantial downtime. For example, in most instances, particulates in the produced fluids cause (1) severe errosion of the well tubing and other production equipment; (2) partial or complete clogging of the flow from the well which requires workover of the well; (3) caving in the formation and collapse of the well casing; (4) extra processing of the fluids at the surface to remove the particulates; and (5) extra cost in disposing of the particulates once they have been separated. Accordingly, it is extremely important to control the production of particulates in most operations.
Probably the most widely-used technique used to control the production of particulates (e.g. sand) from a well is known as gravel packing. In a typical gravel pack, a screen is lowered into the wellbore and positioned adjacent the interval of the well which is to be completed. Particulate material, collectively referred to as gravel, is then pumped as a slurry down the tubing on which the screen is suspended. The slurry exits the tubing above the screen through a "cross-over" or the like and flows downward in the annulus formed between the screen and the well casing or open hole, as the case may be. The liquid in the slurry flows into the formation and/or the openings in the screen which are sized to prevent the gravel from flowing therethrough. This results in the gravel being deposited or "screened out" in the annulus around the screen where it collects to form the gravel pack. The gravel is sized so that it forms a permeable mass around the screen which allows flow of the produced fluids therethrough and into the screen while blocking the flow any particulates produced with the formation fluids.
One of the major problems associated with gravel packing, especially where long or inclined intervals are to be completed, arises from the difficulty in distributing the gravel over the entire interval to be completed, i.e. completely packing the annulus between the screen and the casing in cased wells or between the screen and the wellbore in open hole or under-reamed completions. This poor distribution of gravel (i.e. incomplete packing of the interval) is often caused by the liquid in the gravel slurry being lost into the more permeable portions of the formation interval which, in turn, causes the formation of gravel (e.g. sand) "bridges" in the annulus before all of the gravel has been placed. Such bridges block further flow of the slurry through the annulus which prevents the placement of sufficient gravel (a) below the bridge for top-to-bottom packing operations or (b) above the bridge, for bottom-to-top packing operations.
U.S. Pat. No. 4,945,991 discloses a method for gravel packing an interval of a wellbore wherein there is good distribution of the gravel throughout the desired interval even if sand bridges form before all the gravel is deposited. In this method, perforated shunts or conduits are provided along the external surface of the screen which are in fluid communication with the gravel slurry as it enters the annulus in the wellbore adjacent the screen. If a sand bridge forms before all of the gravel is placed, the slurry will flow through the conduits and out into the annulus through the perforations in the conduits to complete the filling of the annulus above and/or below the bridge. In some instances, valve-like devices are provided for the perforations in these conduits so that there is no flow of slurry through the conduits until a bridge is actually formed in the annulus; see co-pending U.S. Pat. application No. 07/648,061, filed Jan. 31, 1991 and assigned to the present Assignee.